Immunoglobulins (antibodies) are necessary to defend against infection, poisoning, and other potential threats to the health and wellbeing of vertebrates. The presence of immunoglobulins at the interface of a vertebrate organism and the outside environment is the front line of the defense system. Examples of such interfaces include, but are not limited to, the oral cavity and the rest of the digestive system, the nasal cavities and associated respiratory system, the urogenital system, and the eyes. In these structures, antibodies are present in mucosal and other secretions.
Embryonic vertebrates are immunologically naïve. Connor et al (Conner, G. H., M. Richardson, et al. (1977) “Immune responses of the bovine fetus” J Dairy Sci 60(2): 289-93) report that ungulates, calves and lambs, are capable of an immune response when challenged by in utero vaccination. In the absence of such direct challenge by antigens in the sterile uterine environment, fetal ungulates produce no antibodies. In order to survive, therefore, various mechanisms have evolved to provide new born animals with antibodies to protect them after birth, said mechanisms usually involve passively transferring maternal antibodies to the offspring. In placental mammals, classified in the taxon Eutheria, the method of transfer of antibody to the offspring is dependent upon the interface between the placental and the uterine tissues. For example, the hemochorial placentae of primates and rodents allow for passage of immunoglobulins across the placenta to the embryo in utero. At birth antibody protection is present. In animals with endotheliochorial placental interface, such as dogs and cats, limited prenatal immunoglobulin passage occurs through the placenta and must be supplemented by antibody transferred to the neonate after birth. In epitheliochorial placental animals, such as cattle, pigs, horses, deer, and a vast number of related species, no transplacental transfer of immunoglobulin occurs and all antibody protection must be acquired by the neonate after birth.
The primary mechanism of postnatal passive transfer of immunity is neonatal ingestion of colostrum, the “first milk”, shortly after birth. This fluid is produced by the mammary glands before parturition and is extremely high in immunoglobulins. Upon ingesting colostrum, immunoglobulins are absorbed from the neonatal gut and transferred into the circulatory system wherein they function as antibodies in the nascent immune system. In species that depend upon passive transfer of immunoglobulin antibodies from colostrum, such transfer only occurs for a relatively short time, typically about 12-36 hours, during which time the intestine is permeable. After that time, transfer of immunoglobulins does not occur and, therefore, additional passive transfer of immunity from any ingested materials becomes impossible. This mechanism of preparing the newborn for life outside of the uterus is found in a large and ubiquitous group of mammals known as ungulates.
Ungulates are generally hoofed mammals but include some modern species which have hoofed ancestors. Ungulate species include many domesticated animals such as cattle, goats, sheep, horses, and swine as well as non-domesticated animals used as human food such as deer, antelopes, whales, etc. Ungulates have epitheliochorial placentae, vide supra, and, therefore, depend upon postnatal passive transfer of immunity. In regard to passive transfer of immunity to off-spring, the placental structure and prenatal function dictates a high degree of similarity among ungulate species in contrast to other mammalian species with different placental-uterine interface structures.
Ungulate immunology is similar to that of other mammals with immunoglobulin classes or isotypes IgG, IgM, serum IgA, and IgE (Cruse, J. M. and R. E. Lewis (2002) Illustrated Dictionary of Immunology, CRC Press). Immunoglobulin G isotypes of IgG1 and IgG2 are present in ungulate serum. Similar to other mammalian species, IgA predominates at mucosal surfaces. Cruse and Lewis (ibid) also report that products of the preparturient ruminant mammary gland are “different in immunoglobulin content compared to other species.” Colostrum contains a high concentration of IgG and lower concentrations of IgM and IgA. Therefore, immunity is afforded to the neonatal ungulate primarily by IgG and usually of one isotype. For example, in the bovine, IgG1 is the predominant immunoglobulin in colostrum and, therefore, in the circulation of the calf after successful passive transfer of immunity.
Failure of passive transfer (FPT) of immunity is a potentially life-threatening condition to the neonate. FPT may occur due to a number of causes alone or in combination including: inadequate supply of colostrum, low quality colostrum, failure of the neonate to suckle, failure to absorb ingested immunoglobulins from the gut, metabolic breakdown of the antibodies in the neonate, and others. Those skilled in the art are well aware of the danger of FPT to neonatal animals and have developed procedures, practices, tests, and remedies to prevent, detect, or counter the risk. Despite awareness and various procedures, FPT remains a significant problem in raising livestock and other animals around the world. For example, package inserts accompanying FPT testing products, VMRD (VMRD, Inc. P.O. Box 502, Pullman, Wash. 99163, U.S.A.) teach that more than 10% and up to 40% of dairy and beef calves fail to absorb adequate levels of immunoglobulins.
Passive transfer of immunity occurs along a continuum from no transfer to acceptable transfer of antibodies and therefore protection from infection for the neonate. The majority of newborn ungulates receive sufficient antibody to be considered normal and to thrive with normal husbandry including that supplied by the dam. At the lower end of the passive transfer continuum are those animals receiving no antibodies that will not survive without intervention. Between these two extremes are those individuals which would benefit from supportive treatment to maximize thrift, i.e., healthy and vigorous growth. The term “failure of passive transfer” or “FPT” describes less than normal or sufficient transfer of antibody to an individual during the first few feeding postpartum. FPT is also sometimes referred to in the art as “partial FPT”.
Current practice for detecting FPT is to draw a blood sample within a day or two of birth, depending on the animal species being tested, and determine total protein or immunoglobulin in the sample. If present, the concentration of antibody protein is estimated or measured and compared to levels established as sufficient for successful transfer of immunity for that species. Testing methods include measuring total serum protein by refractometry or precipitation with zinc sulphate and measuring immunoglobulins in the blood by various immunoassay methods such as latex agglutination, radial immunodiffusion (RID), enzyme linked immunosorbant assay (ELISA), lateral flow rapid tests, and others. Blood is drawn to perform these test and in most cases blood cells must be removed prior to performing the test. Thus, existing methods for determining amounts of immunoglobulin in neonatal ungulates are time consuming, labor intensive and require obtaining, transporting and storing blood samples. There is accordingly a need for alternative and improved methods for determining immunoglobulin content in neonatal ungulates.